The present invention relates to a semiconductor device and, in particular, to the structure of a ball grid array (hereinafter referred to as BGA) package of the cavity down type, taking heat radiation into account.
As a conventional technology in this field, for example, the following technology has been proposed. FIG. 16 is a plan view of a conventional cavity down type BGA package taking heat radiation into account and FIG. 17 is a cross sectional view taken on a line Exe2x80x94E in FIG. 16.
For the purposes of making an electric wiring circuit and taking heat radiation into account, for example, a stepwise drawn substrate 1 made of a copper plate or the like is prepared, as shown in these drawings. An IC chip 3 is bonded to the drawn portion 1a of the stepwise drawn substrate 1 via an adhesive 2. The electrodes 4 of the IC chip 3 are electrically connected, or wire-bonded, to the electrodes 5 of the stepwise drawn substrate 1 with metal thin wires 6. Then, a sealing material 7 such as epoxy resin or the like is applied thereto and solder balls 8, which are to be terminals, are bonded like a grid to the surface 1b of the substrate by a heating step.
As is shown in FIG. 18 to FIG. 20, the conventional BGA package described is bonded to the substrate of a device, that is, a so-called mother board 9. If this BGA package is exposed to an environment having variations in temperature, stress is concentrated between the solder balls 8 and the drawn substrate 1 by the difference in the thermal expansion coefficient between the mother board 9 and the drawn substrate 1, as shown by arrows in FIG. 19, and may produce a crack. Finally, as shown in FIG. 20, a solder ball 8 may become completely separated from the drawn substrate 1.
This stress is generally concentrated on the corner of the solder ball 8. Also, the stress increases from the center of the BGA package to the outer side because of the geometric moment of inertia. If the solder ball 8 is separated from the substrate 1 in this manner, there is presented a problem in that electric conduction is interrupted.
Also, in order to solve this problem, making the substrate of the BGA package side thin or soft has been tried, but various problems have been presented: that is, the substrate is apt to be warped or deformed; it is difficult to draw the substrate; and when a semiconductor device is mounted on a mother board, solder balls are floated to bring faulty connection between them.
In view of the problems described above, the present invention has been made, and it is the object of the present invention to provide a semiconductor device capable of ensuring adequate bonding strength without applying excessive stress to the solder balls, and having a high degree of reliability.
In order to solve the above problems, according to a first aspect of the present invention, there is provided a semiconductor device comprising a substrate made of metal for the purposes of making an electric wiring circuit and taking heat radiation into account, in which the substrate has recessed portions on one surface opposite to the other surface to which solder balls are bonded.
The recessed portions may be gradually deeper from the center to the outer side. Also, in the above semiconductor device, the recessed portions may be formed in the regions on the diagonals of the substrate. Also, in the above semiconductor substrate, the recessed portions may be formed like steps on the whole surface of the substrate such that they are gradually deeper from the center to the outer side.
Further, in accordance with the second aspect of the present invention, there is provided a semiconductor device comprising a substrate made of metal for the purposes of making an electric wiring circuit and taking heat radiation into account, in which the substrate is made thin at the portion where solder balls are bonded and in which resin is applied to the thin portion of the substrate.
In the above semiconductor device, the thin portion of the substrate may be recessed. Also, in the above semiconductor device, the recessed portions may be formed like steps on the whole surface of the substrate such that they are gradually deeper from the center to the outer side.
According to the third aspect of the present invention, there is provided a semiconductor device comprising a substrate made of metal for the purposes of making an electric wiring circuit and taking heat radiation into account, in which a through hole is made in the center of the substrate and in which a heat radiation plate is bonded to the substrate.
Further, in accordance with the fourth aspect of the present invention, there is provided a semiconductor device comprising a substrate made of metal for the purposes of making an electric wiring circuit and taking heat radiation into account, in which a through hole is made in the center of the substrate and in which an IC chip is fixed in the through hole and in which the reverse surface of the IC chip is exposed outside.
In this connection, in the above semiconductor device according to the first to fourth aspects of the present invention, the substrate may be shaped like steps or subjected to drawing.